1. Field of the Invention
The present invention relates to a communication device and its antenna module, and more specifically, to an antenna module having a ring structure.
2. Description of the Prior Art
In modern society, data is required to be accessible anytime and anywhere. As such, wireless communication devices are the best choice. As technology progresses, portable wireless communication devices such as mobile phones and personal digital assistants (PDA) play an important role in modern life.
In each wireless communication device, the antenna used for receiving and transmitting radio waves is an important component. Especially in a portable wireless communication device, an antenna is not only required to be compact in size, but also required to have a larger bandwidth as the integration of radio data signals (bits per unit time) increases.
As wireless communication technology progresses, access points (AP) are widely used. Computers in a wireless local area network (LAN) needs wireless LAN cards to transmit radio signals, and APs receive the radio signals from each station and bridge the LAN to an Ethernet™ in order to share the resources on the net. In order to expand a wireless network, it is required that a plurality of APs be disposed, or the positions of APs are well arranged in cooperation with a two-layered Ethernet™ exchanger and CAT-5 cables.
Please refer to FIG. 1 showing a network topology using an AP 50. The AP 50 includes an antenna unit 52 for receiving and transmitting radio signals. The operational range of the AP 50 is shown by a broken circle in FIG. 1. When there are three stations accessing the network via the AP 50, the data transmission speed of the AP 50 is shared between these three stations. That is, if the maximum transmission speed of the AP 50 is 11 Mbps, ideally the bandwidth available to each station is only 11/3 Mbps, in other words, the three stations share a bandwidth of 11 Mbps.
In order to improve the data transmission speed or expand the operational range, it is possible to dispose a plurality of APs 50. Please refer to FIG. 2 showing a network topology using three APs 50. By turning on three APs 50, the transmission speed is ideally increased to three times the transmission speed provided by one AP 50. That is, if the maximum transmission speed of the AP 50 is 11 Mbps, ideally three APs 50 provides a bandwidth of 11*3=33 Mbps. Thus, if there are three stations in the network, ideally the bandwidth available to each station is 33/3=11 Mbps. Therefore, the transmission speed can be improved by increasing the number of the APs 50. However, when increasing the number of the APs 50, if different APs 50 use the same or neighboring channels, the main lobe of a channel will overlap with the main lobe or the side lobe of another channel so that interference may occur. Thus in FIG. 2, according to the 802.11b standard, the three APs 50 use channel 1, channel 6 and channel 11, respectively so that the interference due to the overlap of the main lobes is minimized. Please refer to FIG. 3 showing the distribution of channels according to the IEEE 802.11b standard. In the band between 2.400 GHz and 2.484 GHz, the peak of the main lobe of channel 1 is at 2.412 GHz, the peak of the main lobe of channel 6 is at 2.437 GHz, and the peak of the main lobe of channel 11 is at 2.462 GHz, so that there is a bandwidth of 25 MHz between each two of the three channels. The interference caused by the overlap of the main/side lobes can be reduced if the interval used between each channel is kept over 5. Therefore, if there are too many APs 50, the interference between channels becomes more serious and the APs cannot be too close to each other. Accordingly, the network is unable to provide the stations a better transmission speed. Moreover, since each AP requires a network line to be connected to the LAN, the installation is very complicated.